1. Field of the Invention
This invention relates to an improved method of preparing pentaerythritol esters of rosin. In particular, the improvement of this invention lies in reacting the rosin and pentaerythritol in the presence of very low quantities of phosphinic acid (H.sub.3 PO.sub.2), as catalyst, to reduce the reaction time for rosin pentaerythritol ester formation. Also, when tall oil rosin is esterified, a rosin pentaerythritol ester of improved color is produced.
2. Description of the Prior Art
Rosin is mainly a mixture of C.sub.20, fused-ring, monocarboxylic acids, typified by levopimaric and abietic acids, both of which are susceptible to numerous chemical transformations. The rosins to which this invention relates include gum rosin, wood rosin and tall oil rosin.
The natural separation and gradual conversion of some of the hydrophilic components of sap and related plant fluids from the cambium layer of a tree into increasingly hydrophobic solids are the generic process of forming diverse gums, resins and waxes. The oleoresin intermediate in this process is typified in pine gum, which flows from hacks on the trunks of southern yellow pine in southeastern United States, in France, and in other countries. Pine gum contains about 80% (gum) rosin and about 20% turpentine.
Resinification from oleoresin can result from either natural evaporation of oil from an extrudate or slow collection in ducts in sapwood and heartwood. Pinus stumps are valuable enough to be harvested, chipped, and extracted with hexane or higher-boiling paraffins to yield wood rosin, wood turpentine, and other terpene-related compounds by fractional distillation. In the kraft, i.e., sulfate, pulping process for making paper, pinewood is digested with alkali producing crude tall oil and crude sulfate turpentine as by-products. Fractionation of the crude tall oil yields tall oil rosin and fatty acids.
The chemical transformation of gum, wood, and tall oil rosin which relates to this invention is esterification. The beneficial product characteristics provided by rosin esterification for various applications have led to the development of many esterification procedures, particularly treatments with polyhydric alcohols. U.S. Pat. Nos. 2,369,125, 2,590,910 and 2,572,086 teach rosin esterification with glycerol and pentaerythritol, among other polyhydric alcohols, usually preceded by a rosin disproportionation step.
It is generally known in the art that a significant disadvantage of pentaerythritol esterification of tall oil rosin as compared with glycerol esterification is the deterioration of rosin color in the product of the former process. For a tall oil rosin with a starting color of 8 on the Gardner scale, a pentaerythritol ester would have a color of 13-18 while a glycerol ester would have a color of 8-9. Also, extremely long reaction times are required to make the tall oil rosin-pentaerythritol esters (up to 30-48 hours) as compared to making tall oil rosin-glycerol esters under identical conditions (10-12 hours). It was this concern which led to the discovery of the invention process hereinafter claimed.
U.S. Pat. Nos. 3,780,012 and 3,780,013 acknowledge that tall oil rosin, as opposed to gum or wood rosin, darkens significantly upon pentaerythritol esterification and propose alternative solutions. U.S. Pat. No. 3,780,012 teaches pretreating the rosin with paraformaldehyde followed by distillation prior to the esterification reaction. U.S. Pat. No. 3,780,013 teaches the incremental addition of a phenol sulfide compound during the esterification. The color of the product of these procedures was claimed to be an M (U.S.D.A. scale), equal to 11-12 on the Gardner scale. Also, the patents' examples employed a 20% equivalent excess of pentaerythritol.
U.S. Pat. No. 2,729,660 also acknowledges the darkening effect which common esterification catalysts such as strong acids cause on the product during esterification. The patent teaches the use of 0.5 to 5% of either the aliphatic or aromatic esters of phosphorous acid as a catalyst for the esterification of higher fatty acids or rosin acids, or mixtures thereof. In addition to avoiding appreciable color formation during the esterification, a reduction in reaction time is noted. A distinct disadvantage of this process is the dissociation, during esterification, of the alcohol used to make the phosphite ester catalyst resulting in a disagreeable odor.
U.S. Pat. No. 4,172,070 teaches employing arylsulfonic acid in place of the traditional basic esterification catalysts, such as calcium oxide, to reduce the time for tall oil rosin-pentaerythritol esterification to obtain a rosin ester of improved oxygen stability, color and softening point. This work is confounded, however, by the unusually large amount of pentaerythritol used (35% equivalent excess) which by itself would markedly increase the rate of acid number drop. Products with Ring and Ball softening points of 77.degree. C. to 86.5.degree. C. were obtained. Normal commercial pentaerythritol esters of rosins soften between 95.degree. C. and 105.degree. C.
The object of this invention is to provide a novel method of preparing pentaerythritol esters of rosin. It is a further object of this invention to employ a catalyst which accelerates the rosin-pentaerythritol reaction rate to result in a reduced reaction time. It is a still further object of this invention to permit a reduction in the amount of pentaerythritol employed in the reaction, resulting in reduced cost and higher, more desirable softening points, i.e., from 95.degree. C. to 105.degree. C. It is a specific object of this invention to provide a method of preparing pentaerythritol esters of tall oil rosin exhibiting color equivalent to glycerol esters of tall oil rosin.